1. Field of the Invention
The present invention relates to an image forming apparatus employing an electrophotographic system or the like, and more specifically, to an image forming apparatus provided with a plurality of fixing means for heat-fixing unfixed toner on a recording material on a path for transporting the recording material, thereby being capable of passing the recording material through the fixing means on an upstream side with respect to a transport direction of the recording material and then passing the recording material through the fixing means on a downstream side with respect to the transport direction.
2. Related Background Art
Image forming apparatuses, such as printing machines, copying machines, and printers, are becoming to be required to output images having higher image quality. One indicator of image quality of images is glossiness of printed images. In particular, in the case of images such as photographs and illustrations, there is such a tendency that images having higher glossiness are more preferable.
Image glossiness is determined by fixing conditions, such as fixing times, fixing temperatures, fixing widths, and fixing pressures, of fixing apparatuses for fixing unfixed toner images onto sheets such as paper and OHP films. Depending on the fixing conditions at the time of fixation, fused states of the toner and permeability of the toner with respect to the sheets vary, which in turn changes the image glossiness. In ordinary cases, the image glossiness is enhanced as the fixing time is elongated, as the fixing temperature raises, as the fixing width is expanded, and as the fixing pressure increases.
However, it is required not only to enhance the glossiness, but also to reduce warmup time of the fixing apparatus and to reduce power consumption thereof.
As a conventional technique, in order to achieve energy saving, a short warmup time, and high glossiness at the same time, an image forming apparatus including a plurality of fixing devices, that are fixing means, is devised. From a viewpoint of energy saving, in such the image forming apparatus, power application to some of the fixing devices may be suspended. For instance, Japanese Patent Application Laid-Open No. H07-271226 discloses an image forming apparatus including a plurality of image heating devices (fixing devices) and reduces power consumption by driving at least one of the image heating devices at a set temperature and not driving each remaining image heating device at the time of standby. Further, Japanese Patent Application laid-Open No. 2002-372882 discloses a construction in which a plurality of image heating devices are provided, power application to at least one of the plurality of image heating devices is turned off when a glossy mode is set, and power is applied to all of the plurality of image heating devices when a non-glossy mode is set. With the construction described above, image heating devices, whose frequencies of use are low, are rarely supplied of power application.
In the case of an image forming apparatus, into which a plurality of image forming jobs during image forming operation, if the construction includes a plurality of image heating means as described above, there occur the following problems.
In an image forming operation, it is preferable, from a view point of power consumption reduction, that a plurality of image heating means are not used, for example, during image forming job execution, application of power to image heating means not used in this image formation is turned off or an amount of the power applied to the image heating means is reduced. However, when a new image forming job that uses all of the plurality of image heating means is inputted during this image forming job, if power application to place the image heating means not used at the time of the input of the new job under an image heatable state is started to maintain the image heating means under the image heatable state, the longer the currently executed job is performed, the longer the time taken before the new job is started becomes, leading to a loss of energy. Conversely, when the power application for obtaining the image heatable state is started at the time of the start of the new job, a wait time is elongated. Therefore, it is desirable that in such a case, a time, during which the image heatable state is maintained, be shortened as much as possible while shortening the wait time.